Blender in game volume control?
Globally change volume with this Python snippet.  Also affects sound actuators.

import aud

device = aud.device()
device.volume = 0.5  # Change this value to whatever


http://www.blender.org/api/blender_python_api_2_76_2/aud.html

Keep in mind that a new device isn't created until you re-start blender, so any changes made in-game will persist until you actually close blender.